Before MRI It Is The Imaging Of

Before We Start To Discuss The Shorten Time We Should Understand How Does
This Machine Work?

MRI
Machine Work By Using A Very Strong Magnetic Field And Radio Wave
To Give Image Of The Inside Body Part

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MRI It Is The Imaging Of Hydrogen Atoms.

Most Of Human Body Is Consist Of
Water Molecule Which Consists Of One Oxygen And Two Hydrogen Atoms (As We Say
Water). In The Nuclei Of Each Hydrogen Atom There Is A Protons And
Neutrons.  The Protons Are Like Tiny
Magnets And Are Very Sensitive To Magnetic Fields.

 

When The Patient Lie Under
Powerful MRI Scanner, The Protons In The Body Line Up In The Same Direction Of
The Magnetic Field, Like The Needle Of Compass, Short Signal Of Radio
Waves Are Sent To The Area Of Interest Of The Body, Knocking The Protons
Out Of Alignment. When The Radio Waves Are Turned Off, The Protons Realign. The
Movement Of Protons Sends Out Radio Signals, Which Are Picked Up By
Coli Receivers.

These Signals Provide Information
About The Exact Location Of The Protons In The Body. They Also Help To
Differentiate Between The Various Types Of Tissue In The Body, Because The
Protons In Different Types Of Tissue Realign At Different Speeds Will Produce
Distinct Signals.

In The Same Way That Millions Of
Pixels On A Computer Screen Can Create Complex Pictures, The Signals From
The Millions Of Protons In The Body Are Combined To Create A Detailed
Image Of The Inside Of The Body.

To Produce An Image From The
Acquired Data Points We Need To Complete A Mathematical Process Called Fast FTheier
Transform Or  FFT.

 

 

 

 

 

Slice Selection Gradient:

The
Slice Selection Gradient Works Like The Same Way As In A Spin Echo Sequence.
This Gradient Needs To Be Activated To Localize A Slice In The Desired Plane Of
Imaging. A Phase Shift Is Created By Activating The Slice Selection Gradient
And Signal Is Lost In This Plane. A Negative Polarity Slice Selection Gradient
Is Then Activated To Rephase The Hydrogen Nuclei (Protons) So After The Slice
Is Localized, The Maximum Signal Can Be Achieved In This Plane.

Phase Encoding Gradient:

This
Gradient Will Be Activated To Force A Phase Shift In The Hydrogen Nuclei In The
Phase Direction. This Will Localize A Slice Of Data To Be Filled In The
K-Space. This Gradient Needs To Be Activated For Each Line Of The K-Space.

Frequency Encoding Gradient:

The Frequency Encoding Gradient Is
Activated During Data Collection. In Order To Maximize The Signal During This
Period Of Time, A Negative Polarity Frequency Encoding Gradient Is Activated
Before The Data Collection To Decrease Signal In The Hydrogen Nuclei. A
Positive Polarity Frequency Encoding Gradient Is Then Activated To Rephase The
Nuclei So Maximum Signal Is Achieved At The Echo Time.

 

Echo Planar Imaging (EPI) Sequences

It Is One Of The Early MRI Sequences And
It Is The Fastest Acquisition Method In MRI (100 ms For Each Slice) Advantage

But With Low Spatial Resolution
Disadvantage.

This Is Done By Using Rapidly Alternating
Phase And Frequency Coding Gradient Activations. This Gives Rapidly Alternating
Sequence A Unique Beeping Sound.  The EPI
Sequence Can Be Collected As A Blipped
Or Non-Blipped
Technique.

Blipped:
The
Blipped Technique Fill The K-Space In A Zigzag Configuration Until All The
Lines Of K Space Is Filled.

 

 

 

Non-Blipped:

The
Non-Blipped Technique Will Fill The K-Space Line By Line.

Both Techniques Will Fill
All  Lines Of K-Space Very Rapidly  Within One TR.

 

  

Spin
Echo EPI:

The Spin Echo EPI Sequence Utilize The Same Phase And Frequency
Activations Is Seen With Other EPI Sequences. The Difference With This Sequence
Lies In What Occurs Before This Unique Gradient Configuration. A 90° Excitation
Pulse Followed By A 180° Refocusing Pulse Is Performed Followed By A Blipped Or Non
Blipped Technique.

 

DW EPI:

The
Diffusion Weighted EPI Sequence Is Very Similar To A Spin Echo Sequence. Both
Use A 90° °Excitation Pulse Followed By A 180° Refocusing Pulse. The Difference
Lies In A Unique Gradient Activation At Different Amplitudes Prior To The EPI
Gradient Configuration. The Amplitude Of These Gradient Activations Are
Controlled By A Parameter Called The B Value. 
The Higher The B Value Is, The More Diffusion That Will Be Seen In The
Image. This Occurs Because The B Value Will Control The Amplitude Of The
Gradient Activation. The Larger The B Value, The Larger The Gradient
Activation.

Gradient
Echo EPI:

The Gradient
EPI Sequence Will Start With A 90° Excitation Pulse And Go Right Into The EPI
Configuration.

 

 

 

 

Gradient Echo
(GRE) Pulse Sequences:

The Gradient Echo Pulse Sequence Was Created To Obtain
Faster Scan Times Than Seen With The Spin Echo Sequences. This Is Done By
Removing The 180° Refocusing Pulse. This Will Reduce The Scan Time.

Conventional Gradient Echo:

The Conventional Gradient Echo Starts Like The
Same Way As The Spin Echo Sequence. A 90° Excitation Pulse Is Performed To Tilt
Hydrogen Into The Transverse Plane. A Phase Encoding Gradient Is Used To
Localize A Spot To Fill In The K-Space. The Frequency Including Gradient Is
Then Activated When Data Collection Occurs. 
The One Difference Between A Conventional Gradient Echo And A Spin Echo
Sequence Is The Loss Of The 180° Refocusing Pulse.

Advantage:

The Advantage Of The Conventional Gradient Echo Is
That It Is Faster Scan Time Can Be Achieved When Compared To The Spin Echo
Sequence. This Is Due To The Loss Of The 180° Refocusing Pulse.

Disadvantage:

Because We Are Not Performing A 180° Refocusing Pulse,
Transverse Magnetization Is Not Rephased Prior To Data Collection. This Will
Only Allows To Collect The Transverse Decay (T2 Decay) In The Form Of Free
Induction Decay. Free Induction Decay Is Susceptible To Imperfections In The
Field Homogeneity And Other Surrounding Protons. This Means That True T2 Contrast
Cannot Be Obtained And Therefore Is Termed T2*Contrast.

 

Conventional Spin Echo:

The
Convention Spin Echo Is The Most Basic Pulse Sequence In The Spin Echo Family. It
Consists Of A 90° RF Pulse Followed By A 180° Refocusing Pulse And Then The
Echo Are Collected.  

Advantage:

This Will Reduce Artifact Visualization In The Images.

It Will Produce Accurate Demonstration Of Tissue
Signal In The Images.

This Occurs Because There Is No Forcing On Multiple
Echoes To Be Collected In A Short Duration.

Disadvantage:

Because There Is Only Collecting One Echo Per TR,
These Sequences Take A Long Time To Complete. If We Had An Image Matrix Of 256
X 256, We Would Need To Run 256 TR To Fill One K-Space.

Fast
Spin Echo:

The Fast Spin Echo Will Start With A
90° Excitation RF Pulse. It Is Then Followed By Multiple 180° Refocusing Pulses. After Each Of These Refocusing Pulses, An Echo
Are Collected. This Means That Multiple Lines Of K-Space Are Filled In One TR.

Advantage:

This
Sequence Will Collect Many Lines Of K-Space In One TR Which Will Reduce The
Scan Time.
This Sequence Will Provide A Good T2 Contrast.

Disadvantage:

Because Producing A Multiple 180° Pulses In Such A Short Period Of Time, Fat Does Not Have Time To
Decay Properly And Will Contain High Signal When We Collect The Echoes. This Will
Cause Fat To Be Bright On T2 Weighted Images. This Phenomenon Is Called J-
Coupling.

Inverse
Recovery:

The Inverse Recovery Sequence Was
Created To Suppress Tissue. This Sequence Performs This Task By Using A 180°
Inverse Radio Frequency Pulse And Waiting A Period Of Time Associated With The T1
Relaxation Time Of The Targeted Tissue. This Of Time Is Called The Inverse
Time.  When Fat Is Being Suppressed From The Image, It Is Called A Short
Tau Inverse Recovery (STIR) Series. When Cerebrospinal Fluid Is Being
Suppressed From The Image, It Is Called A Fluid Attenuation Inverse Recovery
Sequence (FLAIR).

The
180° RF Pulse Tilts Net Magnetism Anti-Parallel. By Waiting A Period Of Time
Where A Targeted Tissue To Reaches The Transverse Plane, The 90° RF Pulse
Excites Hydrogen, It Will Tilt That Tissue Back To The Inverse Plane
Suppressing That Tissue.

Advantage:

The Advantage Of This Pulse Sequences That Will Get Uniform
Suppression Of The Targeted Tissue.

Disadvantage:

The
Disadvantage Of This Pulse Sequence Is That May Will Have Less Signal-To-Noise
Than Other Suppression Techniques.  The
Reason Is That When It Inverse The Net Magnetism,  It Lose The Signal. As It Recovers, Tilting
That Magnetism 90° At The Excitation Pulse. This Again Will Reduce The Amount
Of Signal Seen In The Image.

Single
Shot:

The Single
Shot Technique Allows A Technologist To Fill An Entire K-Space In One TR. This Occurs
Because Many 180° Refocusing Pulses Are Performed Followed By Enough Echoes To
Fill It. Using This Many 180° Refocusing Pulses May Reduce The Amount Signal
Collected Towards The End Of The ETL. This Means The Image May Be Blurry. This Technique
Is Commonly Used With Half FTheier Technique. This Technique Will Fill Half The
K-Space And Calculate The Rest From This Data. This May Reduce The Scan Time As
Well As Preserve Image Quality.

Advantage:

The Advantage Of This Sequence Is Fast Scan Time. This Technique Is
Very Useful For Cardiac And Breath Hold Techniques.

Disadvantage: 

Due To The
Large Amount Of 180° Refocusing Pulses, Patient Heating May Occur.
Also, The Many Echoes Collected Within One TR May Also Compromise Image Quality.

DRIVE:

This Technique Is Used With Proton
Density And T2 Weighted Imaging. The Sequence Runs Just Like A Fast Spin Echo
Sequence Except Uses A -90° Pulse Right Before The TR Ends.  The Purpose Of This Is To Regain Longitudinal
Magnetization So Maximum Longitudinal Magnetization Will Be Present  Going Into The Next TR. In Fact, The -90° RF
Pulse Is Also The Same Thing As A 270° RF Pulse (360°-90° Equals 270°). This Will
Allows To Force The T1 Recovery Of The Tissue Without Having To Wait A Long TR.

Advantage:

The Advantage
Of This Pulse Sequence Is That Is Able To Achieve T2 And Proton Density
Weighting In The Image Without Having To Wait A Long TR. This Will Reduce The
Scan Time When The Minimum  TR Is Very
Short.

Disadvantage:

This Extra RF
Pulse Will May Cause Heating In The Images.

Hybrid:

The Hybrid Sequence Was Created
To Combine The Best Of Both Worlds (Spin Echo And Gradient Echo). By Using The
180° Refocusing Pulse, True T2 Relaxation Can Be Achieved. The Gradient
Configuration Allows For Multiple Echoes To Be Collected After Each Refocusing
Pulse. This Allows For The Fast Scan Time Seen With Gradient Echo Sequences.

 

Advantage:

This Sequence May Allows To Achieve
Faster Scan Times Than Normal Spin Echo Sequences.
True T2 Contrast Can Be Achieved.

Disadvantage:

This Sequence
Might Be Susceptible To Field Inhomogeneities.

 

Coherent
Steady-State Echo:

Because A
Steady-State Echo Will Produce A Ratio Of T1 To T2 Contrast,  Techniques Need To Be Employed To Isolate A
Specific Image Contrast. Because Residual Transverse Magnetization Is Produced
After Every TR, More Transverse Magnetization Will Be Present Going Into The
Next TR. To Prevent This From Occurring, A Reversal Gradient Is Activated To
Eliminate This Residual Transverse Magnetization. Therefore, T2 Contrast Can Be
Obtained.

Advantage:

Very Fast
Imaging.

Disadvantage:

Produces T2*
contrast.

Incoherent
Steady-State Echo:

Because Of The
Residual Transverse Magnetization Produced In A Steady-State Echo Sequence, T2
Contrast Is Promoted. In Order To Achieve T1 Contrast With A Steady-State Echo
Sequence, A Spoiler Radio Frequency Or Spoiler Gradient Needs To Be Performed.
This Will Eliminate The Residual Transverse Magnetization In The Tissue.

Advantage:

T1 Contrast Is
Produced

Disadvantage:

____________

Steady-State
Free Precession:

The
Steady-State Free Precession Sequence Is Used To Reduce Better T2 Contrast In The
Image. This Is Done By The Use Of A Hahn Echo. A 90° RF Pulse Is Performed
Followed By Another 90° RF Pulse, And Then The Signal Is Collected For That
Slice. Another 90° RF Pulse Is Performed And Another Slice Of Data Is Collected
From This Pulse In The Previous 90° Pulse. And So On.

The Second 90°
RF Pulse Acts Like A Refocusing Pulse Partially Rephasing Are Hydrogen Nuclei.
This Will Help To Achieve True T2 Contrast That Are Imaged.

 Advantage:

Produces More
Accurate T2 Weighting

Disadvantage:

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 How The MR Images Are Formed?

·       
MRI
Machine Ably A strong Magnetic Field To Tissue Target.

·       
Hydrogen
Nuclei/Protons, (In H2O, As We Mentioned Earlier The MR Imaging It Is The
Imaging Of Hydrogen Atoms And Localized Inside The Body ; 70% – 80% Of Human
Body Is Water) Line Up Either In Line Or Opposite Direction Of The Magnetic
Field.

·       
A
Radiofrequency (RF) Wave Pulsed Through Hydrogen Atoms.

·       
RF Wave Is
Shut Off.

·       
Energy
They Absorbed Is Released As Protons Decay, To Their Natural Steady State.

·       
Energy
Is Converted To Digital Data By Using Fast FTheier Transform (FFT) And
Computer Records These Data As Images. 
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The
Common K-Space Approaches Of Shortening The Experiment Time (Filling Methods):

*A Change
Of K-Space Filling Methods Will Reduce The Scan Time.

*K
Space Is Created By:

Activating
The Positive And Negative Polarity Gradient In Both The Phase And Frequency
Direction.

It
Means The Top Half Of K-Space Is A Mirror Image Of
The Bottom Half  But With Different Polarities.

And Also
The Right Half Of K-Space Is A Mirror Image Of The
Left Half  But With Different Polarities.

1.    
Half FTheier
Technique:

Half FTheier Technique Will Reducing The Scan Time By Only Filling The
Half Of K-Space In The Phase Encoding Direction.
The Other Half Of The K-Space  Is Then
Calculated From This Data. This Will Save The Scan Time At The Cost Of Some
Detail.(Top And Bottom Half)

2.    
Partial
Echo Technique:

Partial
Echo Technique Will Reducing The Scan Time By Only Filling The Half Of K-Space
In The Frequency Direction.      The
Other Half Of The K-Space  Is Then
Calculated From This Data. This Will
Save The Scan
Time At The Cost Of Some Detail.(Right And Left Half)

 

 

Why
The MR Images  Takes Such Long Time To
Acquire?

Definition Of The Scan Time

The Time To Complete Data Acquisition Or The
Time To Fill K-Space Lines.

·       
That’s
Mean The Long (Or Short) Of scan Time Depends On The Data Acquisition Of
Filling The K-Space And That’s Why It Is Takes Long Time.

Data
Acquisition Of K-Space:

Data Acquisition In K-Space Means How
To Collect Data For K- Space Over All Slices In A Sequence.

Types Of Data Acquisition:-

1.    
2D Sequential:

2D Sequential Data Acquisition Involves Filling One Line Of K-Space
From Slice One, Then Moving To Slice Two And Filling One Line Of K-Space In,
Etc. After One Line Is Filled In All The K-Spaces In Area Of Interest Covered
By Slices, We Start Filling Line Two In All The K-Spaces And So On.

2.    
2D Volumetric:

2D Volumetric Data Acquisition Involves Filling One Complete K-Space
In Slice One And Then Moving To The Next K-Space In The Next Slice And So On. 

 

3.    
3D Volumetric:

3D Volumetric Data Acquisition Involves Exciting A Volume Of Tissue
And Separating The Slices From It.

 

 

(MRI Acquisition Is Slow Because Lines Need To Be
Acquired One By One To Create One Slice And Multiple Slices Needs To Be
Acquired To Scan A Volume In 3D).

 

The
Relation Between TR, TE, Slice Number And Etc….

·       
TR Controls
The Amount Of Longitudinal Magnetization.                                                     
A Long TR Allows Full Recovery Of The Longitudinal Magnetization. A Long
TR Increases SNR And Short TR Reduces SNR”

·       
A Long TE
Allows Considerable Decay Of The Transverse Magnetization To Occur Before The
Echo Is Collected. While A Short TE Does Not.                                                                     A Long TE Reduces SNR And Short TE Increases SNR”

·       
FA (Flip
Angle) Controls The Amount Of Transverse Magnetization.                                   The Maximum Signal Amplitude Is Created With Flip
Angles Of 90 Degrees.       

A 90 °Flip Angle Maximizes The SNR.

The Lower
The Flip Angle =  The Lower The SNR.

 

Scan Time: TR * NEX * Number Of
Phase Encoding * Number Of Slice Encoding

To Achieve The Shortest Scan Time:

1.    
Shorter
TR.

2.    
CThese
Matrix.

3.    
Minimum
NEX.

 

Increasing The TE Will:

1.    
Decreases
SNR.

2.    
No
Effect On SR(Spatial Resolution).

3.    
Increases
Scan Time.

Increasing The TR
Will:

1.    
Increases SNR.

2.    
No Effect On SR(Spatial
Resolution).

3.    
Increases Scan Time.

Increasing The
Slice Thickness Will:

1.    
Increases SNR.

2.    
Decreases SR(Spatial
Resolution).

3.    
No Effect On Scan Time.

Increasing The Slice Number Will:                                                           

1.    
Increase
The Scan Time

2.    
Increase
TR

3.    
Increase
TE

 

Some
Factors Will Affecting In Reduce The Scan Time:

1.    
Pulse
Sequence:

The
Pulse Sequence That Will Be Chosen Has A Large Role In How Fast The Series Will
Be.

The Conventional Spin Echo Is The Longest Sequence In The Scan Time That Can Be Used. Why?                                                                                                        
Because
Only One Echo Is Collected For Each TR.                                                  
Other Spin Echo Series May Provide Fast Examination Such As The Fast
Spin Echo And Single Shot Technique.  

The Gradient
Echo Family Will Provide Faster Examination
Due To The Loss Of A 180° °Refocusing Pulse.
Steady-state
sequence are
even quicker due To A Very Short TR. 

Echo Planar
Image Is The Fastest Sequence.  This Utilizes A Unique Configuration Of
Gradient Activations That Will Quickly Fill The K-Space Lines.

2.    
Repetition
Time (TR)

Decreasing The TR Will Directly
Control Scan Time And Reduced  It, But
This Is Limited To The Image Contrast Used.

 

3.    
Parallel
Imaging

Parallel Imaging Is Technique Reduce The Scan Time.  This Is Only Possible When Using A Receiver
Coil That Has 2 Or More Coil Elements In The Phase Direction. In This Is The
Situation, The Coils Can Be Used Separately To Fill Multiple Lines Of K-Space  At One Time.                                                                 
One Coil Will Fill Every Other Line Of K-Space And The Other Will Fill
The Other Lines.  This Would Produce 2
Separate K-Spaces , Due To The Way Data Is Collected, A Loss In Signal Will Be
Created.

 

Counts,

4.    
Increase
Bandwidth

A Less Common Way That Able To decrease
The Scan Time Is To Increase The Receiving Bandwidth.  By Doing This We Will reducing  Sampling Time Which Will Allows For A Shorter
TE. This Possibly Will Lower The  TR And Saving
Some Scan Time. 

However This Is Limited To The Image
Contrast That Needed To Produce.

This Method Will Cause Reduce The Signal
To Noise Ratio –SNR .

5.    
Number
of Excitation (NEX)

The Decreasing Of The Number Of
Excitation (NEX) / Averages Will Reduce The Scan Time And Will Decreasing The
Signal To Noise Ratio –SNR.

6.    
Echo
Train Length (ETL)

The Echo Train Length Controls The
Amount Of Echoes Collected Per Single TR.

The More Echoes That are Collected
Per TR Will Reduce The Amount Of Total TR Period That Are Needed, And This Will
Reduce The Scan Time.

Increasing This Factor Will Increase
The T2 Contrast In The Image And Therefore A Limited In Performing T1 Or PD
Weighted Image.

Also, Using Very High Echo Train
Length Values Will  Produce Image
Blurring Why?

Because The Later Echoes Will Not
Contain Enough Signal To Demonstrate Data Properly.

7.    
Phase
Encoding

The Phase Value In The Image Matrix
Will Determine The Amount Of Lines Of K-Space That Need To Be Filled.

The More Lines That Has In The K-Space
Will Require More Echoes To Be Collected. This Will Increase The Scan Time.